The present invention relates to a buoyancy liquid for a Galilean thermometer, a Galilean thermometer comprising this liquid and a method for preparing the same.
Galilean thermometers function in accordance with the principle of buoyancy and are named after the discoverer of this physical mechanism. As essential components, a Galilean thermometer comprises a plurality of floating bodies in a buoyancy liquid surrounding them. As a rule, the floating beads are hollow balls of glass or another inert material as compared with the buoyancy means. Further, they may be dyed or filled with a coloured substance. Further, they advantageously have a number indication which corresponds to a temperature indication.
The buoyancy liquid has a greater thermal expansion than the floating bodies. When the temperature increases, the floating body continuously looses its buoyancy due to a proportional decrease in density of the buoyancy liquid, until the density of the floating body is greater than that of the surrounding liquid at a certain temperature and the floating body sinks. The temperature at which each individual floating body sinks can be adjusted by correspondingly adjusting the weight of the floating bodies in relation to their displacement volume (i.e. their density).
In order to adjust the floating bodies as exactly as possible, it is advantageous if the difference between the thermal expansion coefficient of the buoyancy liquid and the floating body is as great as possible. A buoyancy liquid, which is suitable under this aspect, is ethanol which has a high thermal expansion coefficient.
However, practically, further requirements have to be met by the buoyancy liquid. Galilean thermometers represent temperature measuring equipment which, apart from measuring the temperature, above all things also serve aesthetic purposes and are predominantly used in the private sphere or as an individual note in office rooms. Therefore, the buoyancy liquid must not represent a danger for the environment in the case of unintentionally coming out. The same also applies for the production process of the thermometers, if the buoyancy liquid is openly handled. Consequently, further requirements regarding the buoyancy liquid will have to be met which include: environmental compatibility and non-poisonousness at skin contact, in case of inhalation of the vapours or of unintentional drinking (e.g. by infants). Further, as to avoid fire, the combustibility should be as low as possible.
To meet one or the other of the above-mentioned requirements, carbon tetrachloride or isoparaffin was used in the past instead of ethanol. However, due to its effect regarding the depletion of the ozone layer, carbon tetrachloride has been prohibited. The isoparaffins have an undesired toxicity when they come into contact with the skin and when they are swallowed and, additionally, they have unfavourable effects on the environment and are thus not preferred.
As a further alternative, ethanol had been used in the past which was mixed with water to decrease the low flash point of pure ethanol. Thereby, the flash point could be raised to a level which is relatively harmless. However, mixtures of ethanol/water have the disadvantage that the difference of the thermal expansion coefficients is low, and, thus, it is difficult to adjust the floating bodies.